The invention relates to a method of manipulating sheets in a gathering machine and to a machine which can be used for the practice of the method. More particularly, the invention relates to improvements in a method of manipulating sheets in a gathering machine of the type wherein one or more sheet feeding units or feeders serve to supply successions of sheets into a predetermined path which is defined by a collating conveyor whereon the sheets are accumulated into stacks of overlapping sheets or into similar formations. A gathering machine of the class to which the present invention pertains is disclosed, for example, in commonly owned U.S. Pat. No. 3,414,257 granted Dec. 3, 1968 to Hans Muller.
German Offenlegungsschrift No. 34 24 665 discloses a method according to which sheets are collated to form a series of groups of n sheets each. When the gathering machine which is used for the practice of the method is set up for the gathering of a different series of groups of sheets, the foremost sheets which issue from the feeders are monitored to ascertain their thickness and the monitored thickness is used as a reference value for comparison with monitored thicknesses of the next-following sheets which are being supplied by the respective feeders. The average thickness of the first n sheets which are supplied by each of the feeders is ascertained by an evaluating unit and the thus obtained average thickness is used as a reference value for comparison with monitored thicknesses of the next-following n sheets. In calculating the average thickness, the evaluating unit disregards the missing sheets, if any. If the foremost sheet of the first series of sheets which are being supplied by a particular feeder is missing, all next-following sheets of the same groups are designated as defective (their thickness deviates from the (zero) thickness of the non-existing or missing foremost sheet of the respective group) so that all next-following sheets of the same group are discarded. Alternatively, if the foremost sheet of the first series or group is defective (i.e., its thickness is excessive or unsatisfactory), all sheets of the first series or group which exhibit the same defect are retained in the machine and are processed as satisfactory sheets.
Gathering machines of the type suitable for practice of the method which is disclosed in the aforementioned Offenlegungsschrift are often called upon to gather groups of twenty or more sheets into books, brochures or pamphlets. The likelihood that the machine would turn out unsatisfactory accumulations of twenty or more sheets each is most pronounced after a change of setup, i.e., when the machine is restarted to proceed with the gathering of different sheets or different numbers of sheets in order to form a series of stacks or similar accumulations. At such time, the individual feeders (such feeders normally comprise suction cups or like devices serving to withdraw successive sheets from a magazine and to deliver the withdrawn sheets to an opening device which drops them onto a collating conveyor) are most likely to deliver unsatisfactory sheets, not to deliver any sheets or to simultaneously deliver two or more sheets. The method which can be practiced in accordance with the teaching of the aforementioned Offenlegungsschrift does not take into consideration the fact that the feeders are more likely to operate unsatisfactorily immediately after a change of setup. Therefore, the machine which is operated in accordance with such method is likely to turn out large numbers of unsatisfactory accumulations of sheets or to discard large numbers of satisfactory sheets. The reason is that the method merely provides for detection of the absence of sheets at the start of a gathering operation but it does not take into consideration the possibility that the foremost item which is supplied by a feeder consists of two or more coherent sheets. The delivery of several superimposed sheets in lieu of discrete sheets can be detected only visually, namely by arresting the machine upon completed withdrawal of the foremost sheets from all of the feeders. If the visual detection is dispensed with and a feeder happens to supply two or more coherent sheets as the first item of a series of n successive items, all satisfactory sheets which follow the foremost item (two or more coherent sheets) are automatically ejected as unsatisfactory since their thicknesses deviate from the thickness of the foremost item. In other words, the evaluating means which is associated with a feeder that has delivered an unsatisfactory item (e.g., two coherent sheets) as the foremost item of a series of n items does not receive and store an acceptable reference signal and the starting routine must be repeated until all of the feeders deliver satisfactory foremost items (i.e., discrete sheets of predetermined thickness). A drawback of such method is that the starting routine must be repeated a number of times, especially if the machine operates with a large number of feeders, which entails huge losses in output and the ejection of large quantities of valuable sheet material.
A frequent cause of unsatisfactory feeding of sheets immediately after the gathering machine is started following a change of setup is improper adjustment of sheet withdrawing and/or separating (singularizing) devices (e.g., needles, nozzles which discharge compressed air, suction cups and the like). If at least one of these devices is improperly adjusted, the machine cannot be started at all because the evaluating unit for the corresponding feeder cannot receive and store an acceptable reference signal.